The semiconductor device industry has a market driven need to reduce IC device failures at electrical test, and to improve the operational lifetimes of IC devices. Reduced device failures may result in increased IC fabrication yield and improved device operational lifetime. Increased IC fabrication yields may result in decreased IC prices, and improved market share.
One method to reduce the number of device failures is to analyze failed devices and determine the cause of the failure. The failures may be what are known as field failures that occur at customer sites, or they may occur in products that have been sold to consumers. The failures may be found during wafer level testing at the end of wafer fabrication, or in testing after a supposedly good IC die is placed in a package, or in testing after a supposedly good IC package is placed in a printed circuit board (PCB).
It is known to examine failed devices by means of electrical testing, optical microscopes, transmitting electron microscopes (TEM), scanning electron microscopes (SEM), focused ion beams (FIB), and other well known methods. If, for example, a particle is found that produces a short between two conductive lines in a signal layer, then action may be taken at the fabrication site to reduce particle levels, and thus increase fabrication yield. This method may be used in cases where the failure, such as the illustrative particle just discussed, is at, or near, the surface of the sample, since the failure may not be otherwise visible in an optical or an electron microscope.
If the cause of the device failure is not on the surface of the sample, it is known to cut or fracture the device at a location near the suspected failure site, set the fracture surface in a holding mechanism, such as epoxy, and grind or polish the exposed lateral edge down to approximately the failure location. The location beneath the device surface may then be seen in cross section by SEM or optical microscope, and the nature of the defect may be observed.
Another method to reduce the number of device failures is to examine test structures fabricated along side the production semiconductor wafers to determine if each production step has been completed within the specified tolerances. These tests may be electrical or physical, and may be destructive tests, such as physically measuring the amount of pressure required to pull a metal layer off of the surface. It is also known to make such destructive tests on small portions of production wafers, such as by using the non-functional area between IC die, which may be known as scribe lines or streets, to form the test structures. However, such tests may not accurately reflect the actual situation on the production IC die, since the scribe lines may not be treated exactly the same as the IC die, for example in not having the same area density of dielectric as a real circuit area.
Another method of testing to determine if each production operation has results that are within the specified range, is to deconstruct or reverse process a small area on a production IC die. For example, it is known to remove the top layers of an IC device by means of what may be known as a spot etch, in which a small elastomeric ring formed of a chemically resistant material is pressed onto the surface of the IC in the area of the suspected defect and serves to hold an etching solution designed to selectively remove some or all of the top layers of the structure and expose potential defects. For example, it is possible to use a chemical solution that preferentially attacks oxide layers to remove the passivation oxide over a polysilicon structure, and determine by SEM examination if a non-oxide dielectric layer under the polysilicon has been properly etched. However, the size of the elastomeric ring is very large as compared to the dimensions of typical IC structures, and may be larger than 2 mm in diameter, and thus produces a relatively large hole in the IC device. Further, there is no method to image the surface during the material removal process to determine if the lateral positioning is correct, or to determine if the depth of the material removal has reached the desired location. Thus, the spot etch, or deconstruction etching, must be done using timed etches and assumed or estimated etch rates. The use of liquid etch materials also limits the size of the hole that may be etched, since very small holes may have problems with reactant exchange with the bulk of the etch media, reactant depletion, and surface wetting problems including bubble formation. Such wetting, depletion, and other etch initiation problems, also contribute to the variability in etch depth.
It is known to etch small diameter holes of several microns in diameter in IC surfaces by means of what may be known as ion milling, using focused ion beams of such heavy ions as gallium. It is possible to analyze the material being etched by examining the atoms in the exhaust gas, typically using optical emission, atomic absorption, infrared, Raman, or mass spectroscopy. However, ion milling is generally not selective etching of materials, such as oxide over metal or polysilicon as in the previously discussed example. Such selective etch rates may be referred to as having an etch ratio. Ion milling may be compared to the use of a drill, cutting everything in its path relatively equally versus the high selectivity available with the chemical spot etching discussed above.
A method is needed to chemically etch a small area with high selectivity between different material etch rates, and the ability to observe the etching. The ability to analyze the composition of the material being etched would also be beneficial in determining the cause of a failure. With such an arrangement, the sample may be imaged during the small spot etching, and the etching may continue until the desired structure is completely exposed and ready for testing. Further, analysis of materials that may appear unexpectedly during the progress of etching may also be performed.